In general, it is well known in papermaking that the drainage of liquid from the paper stock on the fabric is an important step to insure a quality product. This is done through the use of drainage blades or foils usually located at the wet end of a Fourdrinier paper machine. (Note the term drainage blade as used herein is meant to include blades or foils that induce drainage or stock activity or both.) A wide variety of different designs for the blades is available. Typical blades provide for a bearing surface for the wire or fabric with a trailing portion for dewatering which angles away from the wire. This creates a gap between the blade surface and the fabric which causes a vacuum between the blade and the fabric. This not only drains water out of the fabric but also can result in pulling the fabric down. When the vacuum collapse the fabric returns to its position which can result in a pulse across the stock which may be desirable for stock distribution. The activity (caused by the wire deflection) and the amount of water drained from the sheet are directly related to vacuum generated by the blade (and therefore) to each other. Drainage and activity by such blades can be augmented by placing the blade or blades on a vacuum chamber. The direct relationship between drainage and activity is not desirable since while activity is always desirable, too much drainage early in the sheet formation process may have adverse effects on retention of fibers and filler. Rapid early drainage may also cause sheet sealing making subsequent water removal more difficult. Existing technology forces the paper maker to compromise desired activity in order to slow early drainage.
Drainage can be accomplished by way of a liquid to liquid transfer such as that taught in U.S. Pat. No. 3,823,062 to Ward. This reference teaches the removal of sudden pressure shocks to the stock. It is stated that controlled liquid to liquid drainage of water from the suspension is less violent then conventional drainage.
Similar type drainage is that taught in U.S. Pat. No. 5,242,547 to Corbellini. This patent teaches preventing the formation of a meniscus (air/water interface) on the surface of the forming fabric opposite the sheet to be drained. This reference achieves this by flooding the vacuum box structure containing the blade(s) and adjusting the draw off of the liquid by a control mechanism. It is referred to as "Submerged Drainage". Improved dewatering is said to occur through the use of sub-atmospheric pressure in the suction box.
In addition to drainage, blades are constructed to purposely create activity in the suspension to provide for desirable distribution of the flock. Such a blade is taught for example in U.S. Pat. No. 4,789,433 to Fuchs. This reference teaches the use of a wave shaped blade (preferably having a rough dewatering surface) to create microturbulence of the fiber suspension.
Other type blades wish to avoid turbulence yet effect drainage such as that described for example in U.S. Pat. No. 4,687,549 to Kallmes. This reference teaches filling the gap between the blade and the web. It is said that the absence of air prevents expansion and cavitation of the water in the gap and substantially eliminate any pressure pulses.
A number of other blades and arrangements can be found in the following prior art.
______________________________________ 5,393,382 5,089,090 4,838,996 5,011,577 4,123,322 4,909,906 3,874,998 4,459,176 3,598,694 4,425,189 4,544,449 3,922,190 5,437,769 3,870,597 5,389,207 3,738,911 5,387,320 5,169,500 ______________________________________
Present high and low speed paper machines produce different grades of paper with a wide range of basis weights. Sheet forming is a hydromechanical process and the motion of the fibers follow the motion of the fluid because the inertial force of an individual fiber is small compared to the viscous drag in the liquid. Formation and drainage elements effect three principle hydrodynamic processes, which are drainage, stock activity and oriented shear. Liquid is a substance that responds according to shear forces in or on it. Drainage is the flow through the wire, and its characterized by a flow velocity that is usually time dependant.
Stock activity, in an idealized sense, is the random fluctuation in flow velocity in the undrained fiber suspension, and generally appears due to a change in momentum in the flow due to deflection of the forming fabric in response to drainage forces or as being caused by blade configuration. The predominant effect of activity is to break down networks and to mobilize fibers in suspension. Oriented shear and activity are both shear-producing processes that differ only in their degree of orientation on a fairly large scale, that is, a scale that is large compared to the size of individual fibers.
Oriented shear is shear flow having a distinct and recognizable pattern in the undrained fiber suspension. Cross Direction ("CD") oriented shear improves both sheet formation and test. The primary mechanism for CD shear (on paper machines that do not shake) is the creation, collapse and subsequent recreation of well defines Machine Direction ("MD") ridges in the stock of the fabric. The source of these ridges may be the headbox rectifier roll, the head box slice lip (see International Application PCT WO95/30048 published Nov. 9, 1995) or a formation shower. The ridges collapse and reform at constant intervals depending upon machine speed and the mass above the forming fabric. This is referred to as CD shear inversion. The number of inversions and therefore the effect of CD shear is maximized if the fiber/water slurry maintains the maximum of its original kinetic energy and is subjected to drainage pulses located (in the MD) directly below the natural inversion points.
In any forming system, all these hydrodynamic processes may occur simultaneously. They are generally not uniformly distributed in either time or space, and they are not wholly independent of one another, they interact. In fact each of these processes contributes in more than one way to the overall system. Thus while the above mentioned prior art may contribute to some aspect of the hydrodynamic processes aforesaid they do not coordinate all processes in a relatively simple and effective way.